Fwd: Pd-103 activity verification

[Ccja@aol.com]

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Forwarded message:
From:	CBS970@AOL.COM
Sender:	medphys@LISTS.WAYNE.EDU (Medical Physics Listserver)
Reply-to:	MEDPHYS@LISTS.WAYNE.EDU (Medical Physics Mailing List)
To:	MEDPHYS@LISTS.WAYNE.EDU (Multiple recipients of list MEDPHYS)
Date: 97-10-31 08:45:41 EST

Quoting:
> I do check the activity of Pd-103 seeds. In the absence of an NBS tracable
> standard, all
>  you can really do is demonstrate reproducibility (which is, of course, the

> most
>  important thing).  I've not had great results and am flagging just as hard

> as I can.  As
>  far as how:  Standard imaging HDR well chamber, I-125 ADCL factor, with
> Standard Imaging
>  provided energy dependence factor.  This last part is where the smoke-and-
> mirrors comes
>  in, and makes this a constancy-only check.

This sounds about as good as it gets unless you also calibrate your chamber
with the same type of cadmium source Theragenics uses.  The trouble one runs
into (like 3M did with I-125 seeds in the early 90's) is that slight
differences in construction of the chamber can give large differences in
calibration factor, especially when one isotope (energy spectrum) is used in
multiple chambers as the standard for another isotope (energy spectrum).  At
these low energies the photoelectric interactions vary significantly with
chamber materials.  3M had two chambers of the same model, but purchased some
time apart, and transferred the calibration factor with their Am-241 check
source.  Perhaps the new chamber was colored with a different dopant or
whatever, but the response of the new chamber was very different from the
first when you inserted an I-125 seed.  Had they transferred the calibration
with an I-125 seed I would not have had the exhilarating experience of
experience of visiting St Paul, Minnesota in the dead of winter with a
record-breaking cold front passing through (after I had lost most of my cold
tolerance living in Houston, Texas).  Absent of a directly NIST-traceable
Pd-103 calibration let us hope that only the original chamber is calibrated
with the Cd source and all others with a Pd seed calibrated in the original
chamber.
   If NIST changed the calibration factor on the Cd source, then that's
another detective story.
             -- Carter Schroy


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